1. Field of the Invention
The present invention relates to an optical head apparatus for reproducing or deleting information recorded on an optical medium such as an optical disk and an optical card, or recording information onto the optical medium. The present invention also relates to a light source apparatus and an optical element used in such an optical head apparatus. Furthermore, the present invention relates to an optical information processing apparatus including an optical head apparatus.
2. Description of the Related Art
In recent years, with the development of an optical disk, standards of various kinds of optical disks, such as recording/reproducing optical disks and read-only memory (ROM) optical disks, have been proposed and utilized. Various kinds of different laser beams are used often with respect to these optical disks. In such a circumstance, an optical head is proposed that has light sources with two wavelengths so that, in the same apparatus, information can be recorded onto/reproduced from different kinds of incompatible optical disks using different laser beams. Such a procedure is disclosed by Sadao Mizuno et al. xe2x80x9cOptical head for Integrated DVDxe2x80x9d (National Technical Report Vol. 43, No. 3, June, p. 275 (1997)). In such an optical head, it is required to dispose each light source point on a common optical axis.
As a procedure for realizing the above, optical axes of light beams from two light sources generally are aligned by a dichroic mirror. In order to simplify an optical system, a method for aligning optical axes using a diffraction element also is proposed (JP 11(1999)-110785 A, JP 10(1998)-320814 A, JP 10(1998)-326428A, and JP 10(1998)-319318 A). However, the diffraction element with the above-mentioned configuration generates not only diffracted light but also transmitted light, resulting in a light source with an unsatisfactory light utilization efficiency.
In order to solve the above-mentioned problem, a method using a polarization anisotropic hologram is proposed (JP 11(1999)-161996 A, JP 10(1998)-289468 A).
Hereinafter, a conventional light source and optical head apparatus will be described by illustrating JP 11(1999)-161996 A.
FIG. 16 shows a configuration of a conventional optical head apparatus. Two different semiconductor lasers are provided in a package 119 (described later) so as to emit light beams L1 and L2 with different wavelengths in an upward direction of the package 119 from different points. Furthermore, the light beams L1 and L2 are polarized so that their polarization directions are orthogonal to each other. The light (L1 and L2) output from the package 119 is incident upon a polarization splitting hologram 120. The polarization splitting hologram 120 diffracts light polarized in a particular direction, and transmits light polarized in a direction orthogonal to the particular direction. Herein, the polarization splitting hologram 120 is disposed so as to diffract the light beam L2 and transmit the light beam L1.
The polarization splitting hologram 120 is designed so that the light beam L2 is diffracted, and an apparent emission point of the diffracted light coincides with an emission point of the light beam L1. Thus, the optical axes of the light beams L1 and L2 are aligned. The resultant light is incident upon an objective lens 103, and condensed onto a recording surface of an optical disk 350 by the objective lens 103.
Next, light sources in the package 119 will be described with reference to FIGS. 17A and 17B. FIG. 17A is a plan view seen from an output direction of the package 119, and
FIG. 17B is a cross-sectional view in a plane of the package 119 substantially parallel to an optical axis. Semiconductor lasers 611 and 612 are provided in the package 119, and respectively emit light beams L1 and L2. The semiconductor lasers 611 and 612 are attached to the package 119 via holders 301 and 302 so that the polarization directions of emitted light beams are orthogonal to each other. Light beams emitted from the semiconductor lasers 611 and 612 are reflected in an upward direction of the package 119 by reflective mirrors 641 and 642.
Because of the above-mentioned configuration, a high-efficiency two-wavelength optical head with optical axes aligned can be realized.
Furthermore, JP 11(1999)-161996 A discloses an optical head apparatus using, as the semiconductor lasers 611 and 612, those which oscillate in different polarization directions and are disposed in the same direction. Even with the use of this technique, a high-efficiency two-wavelength optical head apparatus with optical axes aligned can be realized.
However, according to the method of JP 11(1999)-161996 A, it is required to mount the semiconductor lasers 611 and 612 in the package 119 in different directions so that polarization directions of two wavelengths are orthogonal to each other, which results in a complicated configuration and a high production cost.
Furthermore, according to the method using semiconductor lasers oscillating in different polarization directions and disposed in the same direction as disclosed in JP 10(1998)-289468 A, it is difficult to realize a semiconductor laser that oscillates stably in a TE mode, and the production cost thereof also is high.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an inexpensive light source apparatus that generates two light beams with different wavelengths at a high efficiency. Another object of the present invention is to provide a low-cost optical element that is capable of generating two light beams with different wavelengths at a high efficiency. Still another object of the present invention is to provide an optical head apparatus and an optical information processing apparatus using such a light source apparatus. Still another object of the present invention is to provide a method for adjusting positions of emission points in such a manner that apparent positions of light sources of two light beams with different wavelengths are aligned with each other as desired.
In order to achieve the above-mentioned objects, the present invention has the following construction.
A light source apparatus according to a first construction of the present invention includes: a first light source for emitting light having a first wavelength and polarized in a first direction; a second light source for emitting light having a second wavelength different from the first wavelength and polarized in the first direction; a wavelength plate that functions substantially as a (2m+1)xcex/2 plate (m is an integer) with respect to light having the first wavelength, and functions substantially as a nxcex plate (n is an integer) with respect to light having the second wavelength; and a diffraction element that diffracts light polarized in the first direction and transmits light polarized in a second direction orthogonal to the first direction, wherein the wavelength plate converts light emitted from the first light source into light polarized in the second direction so as to transmit it, and transmits light emitted from the second light source without converting a polarization direction thereof, and the diffraction element diffracts light emitted from the second light source, passing through the wavelength plate, whereby the light emitted from the second light source, diffracted by the diffraction element, has an apparent emission point different from an emission point of the second light source. According to this construction, a light source apparatus can be provided at a low cost in a simple configuration, which is capable of emitting two light beams having different wavelengths with optical axes aligned at a high efficiency.
In the above-mentioned first light source apparatus, the apparent emission point of the light emitted from the second light source can be allowed to coincide with an emission point of the first light source. According to this construction, a light source apparatus can be provided that emits two light beams with different wavelengths from an apparently common emission point.
Furthermore, in the above-mentioned first light source apparatus, in the case where the first light source emits light having directivity in a particular direction, the apparent emission point of the light emitted from the second light source can be positioned in the particular direction from an emission point of the first light source. According to this construction, a light source apparatus can be provided that emits two light beams with different wavelengths from two emission points at different positions on an apparently identical optical axis.
The above-mentioned first light source apparatus further may include a second wavelength plate that functions substantially as a (2mxe2x80x2+1)xcex/2 plate (mxe2x80x2 is an integer) with respect to light having the first wavelength, and functions substantially as a nxe2x80x2xcex plate (nxe2x80x2 is an integer) with respect to light having the second wavelength, wherein the second wavelength plate converts the light emitted from the first light source, which is converted into light polarized in the second direction, into light polarized in the first direction. According to this construction, a light source apparatus can be provided that emits two light beams having an identical polarization direction and different wavelengths.
Furthermore, a light source apparatus according to a second construction of the present invention includes: a first light source for emitting light having a first wavelength and polarized in a first direction; a second light source for emitting light having a second wavelength different from the first wavelength and polarized in the first direction; a wavelength plate that functions substantially as a nxcex plate (n is an integer) with respect to light having the first wavelength, and functions substantially as a (2m+1)xcex/2 plate (m is an integer) with respect to light having the second wavelength; and a diffraction element that transmits light polarized in the first direction and diffracts light polarized in a second direction orthogonal to the first direction, wherein the wavelength plate converts light emitted from the second light source into light polarized in the second direction so as to transmit it, and transmits light emitted from the first light source without converting a polarization direction thereof, and the diffraction element diffracts the light emitted from the second light source, passing through the wavelength plate, whereby the light emitted from the second light source, diffracted by the diffraction element, has an apparent emission point different from an emission point of the second light source. According to this construction, a light source apparatus can be provided at a low cost in a simple configuration, which is capable of emitting two light beams having different wavelengths with optical axes aligned at a high efficiency.
In the above-mentioned second light source apparatus, the apparent emission point of the light emitted from the second light source can be allowed to coincide with an emission point of the first light source. According to this construction, a light source apparatus can be provided that emits two light beams with different wavelengths from an apparently common emission point.
Furthermore, in the above-mentioned second light source apparatus, in the case where the first light source emits light having directivity in a particular direction, the apparent emission point of the light emitted from the second light source can be positioned in the particular direction from an emission point of the first light source. According to this construction, a light source apparatus can be provided that emits two light beams with different wavelengths from two emission points at different positions on an apparently identical optical axis.
The above-mentioned second light source apparatus further may include a second wavelength plate that functions substantially as a nxe2x80x2xcex plate (nxe2x80x2 is an integer) with respect to light having the first wavelength and functions substantially as a (2mxe2x80x2+1)xcex/2 plate (mxe2x80x2 is an integer) with respect to light having the second wavelength, wherein the second wavelength plate converts the light emitted from the second light source, which is converted into light polarized in the second direction, into light polarized in the first direction. According to this construction, a light source apparatus can be provided that emits two light beams having an identical polarization direction and different wavelengths.
Next, an optical element according to a first construction of the present invention includes: a wavelength plate that functions substantially as a nxcex plate (n is an integer) with respect to light having a first wavelength, and functions substantially as a (2m+1)xcex/2 plate (m is an integer) with respect to light having a second wavelength; and a diffraction element that transmits light polarized in a first direction and diffracts light polarized in a second direction orthogonal to the first direction, wherein the wavelength plate and the diffraction element are disposed so that an angle formed by an advanced-phase axis of the wavelength plate and the first direction is substantially 45xc2x0. According to this construction, by allowing light emitted from a first light source, having a first wavelength and polarized in a first direction and light emitted from a second light source disposed at a position different from that of the first light source, having a second wavelength and polarized in the first direction, to be incident upon the above-mentioned first optical element, two light beams can be emitted with optical axes aligned without substantially decreasing the intensity of light.
Furthermore, an optical element according to a second construction of the present invention includes: a first wavelength plate that functions substantially as a nxcex plate (n is an integer) with respect to light having a first wavelength, and functions substantially as a (2m+1)xcex/2 plate (m is an integer) with respect to light having a second wavelength; a second wavelength plate that functions substantially as a nxe2x80x2xcex plate (nxe2x80x2 is an integer) with respect to light having the first wavelength, and functions substantially as a (2mxe2x80x2+1)xcex/2 plate (mxe2x80x2 is an integer) with respect to light having the second wavelength; and a diffraction element that transmits light polarized in a first direction and diffracts light polarized in a second direction orthogonal to the first direction, wherein the first wavelength plate, the diffraction element, and the second wavelength plate are disposed in this order so that an angle formed by an advanced-phase axis of the first wavelength plate and the first direction and an angle formed by an advanced-phase axis of the second wavelength plate and the first direction both are substantially 45xc2x0. According to this construction, in addition to the effect of the above-mentioned first optical element, polarization directions of two emitted light beams can be aligned.
Next, an optical head apparatus of the present invention includes: a radiation light source; a converging optical system for converging a light beam from the radiation light source to form a minute spot on an information recording medium; and a photodetector for receiving a light beam reflected from the information recording medium to output a photoelectric current, wherein the radiation light source is the above-mentioned first or second light source apparatus. According to this construction, the radiation light source emits two light beams having different wavelengths with optical axes aligned, so that it is not required to provide a converging optical system and a photodetector in correspondence with light having each wavelength. Thus, a miniaturized optical head apparatus can be provided at a low cost without increasing the number of adjustment processes.
Furthermore, an optical information processing apparatus of the present invention includes an optical head apparatus for irradiating an information recording medium with a light beam and reading information from light reflected from the information recording medium, and an electronic circuit for controlling the optical head apparatus and processing the read information, wherein the optical head apparatus is the above-mentioned optical head apparatus of the present invention, and the electronic circuit controls a light source apparatus in the optical head apparatus so that it emits light having an appropriate wavelength, depending upon the kind of the information recording medium. According to this construction, an optical information processing apparatus can be provided at a low cost in a simple configuration, which is capable of recording and/or reproducing information with respect to an information recording medium using different wavelengths.
Next, a method of a first construction of the present invention for adjusting a position of an emission point, in an optical system including a first light source for emitting light having a first wavelength and polarized in a first direction, and a second light source for emitting light having a second wavelength different from the first wavelength and polarized in the first direction, comprises: allowing light emitted from the first light source and light emitted from the second light source to be incident upon a wavelength plate that functions substantially as a (2m+1)xcex/2 plate (m is an integer) with respect to light having the first wavelength and functions substantially as a nxcex plate (n is an integer) with respect to light having the second wavelength, the wavelength plate converting light emitted from the first light source into light polarized in a second direction orthogonal to the first direction so as to transmit it and transmitting light emitted from the second light source without converting a polarization direction thereof, and allowing the transmitted light to be incident upon a diffraction grating so that the light polarized in the first direction is diffracted and the light polarized in the second direction is transmitted, thereby adjusting a position of an emission point so that the light emitted from the second light source has an apparent emission point different from an emission point of the second light source. According to this construction, two light beams with different wavelengths can be emitted at a high efficiency with optical axes aligned at a low cost in a simple configuration.
In the above-mentioned first method for adjusting a position of an emission point, the apparent emission point of light emitted from the second light source can be allowed to coincide with an emission point of the first light source. According to this construction, two light beams with different wavelengths can be emitted from an apparently common emission point.
Furthermore, according to the above-mentioned first method for adjusting a position of an emission point, in the case where the first light source emits light having directivity in a particular direction, the apparent emission point of light emitted from the second light source can be positioned in the particular direction from an emission point of the first light source. According to this construction, two light beams with different wavelengths can be emitted from two emission points at different positions on an apparent identical optical axis.
Furthermore, a method of a second construction according to the present invention for adjusting a position of an emission point, in an optical system including a first light source for emitting light having a first wavelength and polarized in a first direction, and a second light source for emitting light having a second wavelength different from the first wavelength and polarized in the first direction, comprises: allowing light emitted from the first light source and light emitted from the second light source to be incident upon a wavelength plate that functions substantially as a nxcex plate (n is an integer) with respect to light having the first wavelength and functions substantially as a (2m+1)xcex/2 plate (m is an integer) with respect to light having the second wavelength, the wavelength plate transmitting light emitted from the first light source without converting a polarization direction thereof and converting light emitted from the second light source into light polarized in a second direction orthogonal to the first direction so as to transmit it; and allowing the transmitted light to be incident upon a diffraction grating so that the light polarized in the second direction is diffracted and the light polarized in the first direction is transmitted, thereby adjusting a position of an emission point so that the light emitted from the second light source has an apparent emission point different from an emission point of the second light source. According to this construction, two light beams with different wavelengths can be emitted at a high efficiency with optical axes aligned at a low cost in a simple configuration.
According to the above-mentioned second method for adjusting a position of an emission point, the apparent emission point of light emitted from the second light source can be allowed to coincide with an emission point of the first light source. According to this construction, two light beams with different wavelengths can be emitted from an apparent common emission point.
Furthermore, according to the above-mentioned method for adjusting a position of an emission point, in the case where the first light source emits light having directivity in a particular direction, the apparent emission point of light emitted from the second light source can be positioned in the particular direction from an emission point of the first light source. According to this construction, two light beams having different wavelengths can be emitted from two emission points at different positions on an apparent identical optical axis.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.